Grinding circuit control



1957- R. L. CAMPBELL, JR 3,352,499

GRINDING CIRCUIT CONTROL 5 Sheets-Sheet 2.

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19.67 R. L. CAMPBELL. JR 3, 5

GRINDING CIRCUIT CONTROL Original Filed A ril 4, 1960 3 Sheets-Sheet 5 I NVENTOR United States Patent 3,352,499 GRENDING QIRCUIT CONTROL Ross L. Campbell, In, Rochester, N.Y., assignor to The Industrial Nucleonics Corporation, a corporation of Qhio Continuation of abandoned application Ser. No. 19,665, Apr. 4, 1960. This application Dec. 4, 1964, Ser. No.

4 Claims. (Cl. 24121) This is a continuation of a prior copending application Ser. No. 19,665 filed Apr. 4, 1960 (now abandoned).

This invention relates to the uniformity control of industrial processes and more particularly to an improved method and means for controlling the uniformity of slurry from a wet grinding mill.

Size reduction of quarried or mined products from large stones to pulverulent powder usually requires many steps of crushing and grinding. I aw crushers, rotary crushers, ball mills and pebble mills gradually reduce the particle size of the material to be ground until it is small enough to be easily handled. Although both dry grinding and Wet grinding methods are commonly employed in size reduction processes, surface forces often cause flocculation and cushioning of adjacent particles in dry grinding; however, in wet grinding, Water or other liquid solvents are added to the input of the mill along with the solid material as a transport medium to prevent overgrinding.

The majority of wet grinding systems in use are continuous and either of the open or closed circuit type. In the open type, the solid raw materials are fed into the mill and reduced in one pass to the limiting size. In the closed circuit type, the mill output is classified and oversize particles are recirculated. Regardless of the type, since the ability of a material to withstand reduction generally depends upon its hardness, variations in the hardness of the mill feed affect the uniformity of the ground material. The solid-to-liquid weight ratio of the slurry discharge of the mill changes according to these hardness variations. The mill responds to an increase in ore hardness by increasing the size of grind. The solids concentration of the slurry discharge decreases proportionately. However, if a softer ore is introduced to the mill, the size of grind becomes smaller due to the easier grindability of the ore, and the solids concentration of the slurry becomes correspondingly greater. As a result, the mill does not operate at maximum efiiciency; the variable characteristic of the slurry imposes myriad operating problems on apparatus adapted to further process the material such as classifiers, separators and kilns; excessive wear of the grinding elements is commonplace; and, in general, the greatest recovery per operating dollar is not realized.

In an effort to solve the foregoing problems, the present invention provides a system for controlling the apparent density of slurry produced by a wet-grinding mill. Through the application of the present invention, percent solids variation from the mill is reduced; impact action is automatically adjusted to the hardness of feed to eliminate excessive wear of mill lining and grinding elements; the amount of desirable throughput is increased; and, there is an overall reduction in energy required per unit of product produced.

Particularly, the present invention provides a radiation absorption gauge which analyzes the mill effluent by measuring the apparent density of the slurry. The gauge provides a density-functional signal which is compared with another signal representative of a desired or target density of the slurry. The difference between the two signals comprises an error signal which is applied to a controller. In response to an error signal, the controller adjusts a valve regulating the flow of diluting liquid admitted to the mill input to maintain a solid-to-liquid weight ratio of the slurry which is representative of the desired density.

Accordingly, it is a primary object of the present invention to provide an improved system for controlling a grinding process to produce slurry having a substantially constant solids concentaration.

It is another object of the present invention to provide apparatus for increasing the eiiiciency of a grinding process.

It is still another object of the present invention to provide apparatus for uniformly controlling the size of material produced by a grinding process.

It is yet another object of the present invention to eliminate costly over-grinding of material.

It is a further object of the present invention to provide apparatus which is readily adapted to existing grinding processes with a minimum of modification and exense.

It is an additional object of the present invention to provide a slurry control system utilizing a density transducer which occupies a minimum of space, does not interfere with the normal process flow and requires little maintenance.

These and numerous other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of an open circuit grinding process;

FIG. 2 is a graph illustrat'mg the timewise variations in the density of the slurry product of the grinding process shown in FIG. 1;

FIG. 3 is a block diagram schematically showing the application of the density measuring and control system of the present invention to the process of FIG. 1;

FIG. 4 is a graph showing the improvements in product density eifected by the system shown in FIG. 3;

FIG. 5 is a diagrammatic view showing control of a closed circuit grinding process in accordance with slurry density and mass flow rate measurements;

FIG. 6 is a detailed pictorial view of the flow meter diagrammatically illustrated in FIG. 5;

FIG. 7 is a diagrammatic showing of a cement manufacturing process comprising a preferred embodiment of the present invention;

FIG. 8 is a perspective View of a housing for the density transducer utilized in the various embodiments of the present invention;

FIG. 9 is a transverse section of the housing shown in HG. 8; and

FIG. 10 is a graph showing the response of the density transducer utilized in the present invention.

With reference to the drawings, FIG. 1 shows a typical wet grinding process including a mill 10 for receiving solid raw material to be ground from a source 12 and a liquid to facilitate grinding from a source 14. Flow regulating valves, generally shown at 16 and 18, control the addition of these substances to the mill input A. The mill 1%) may contain hardened rollers, balls or pebbles depending on the nature of the raw material 12 and the final particles size required. The liquid-solid mixture upon which the mill 10 operates is commonly called a slurry. Solids 12 are gradually reduced in traveling through the mill 16 until particles of the desired size are obtained.

Efliuent line 20 conveys the slurry from the mill discharge B.

Referring to FIG. 2, the density variations of the discharged slurry are graphically plotted against time. From the graph, it is seen that the apparent density of the slurry may vary widely between densities p and p The inconsistency of the output is due primarily to the inevi' table changes in the hardness of the solids fed to the mill 10. Since the mill is designed to operate most efficiently upon solids of a given hardness in which case the proper size reduction of the solid takes place with a minimum of power consumed, the mill is unable to cope with hardness changes in the solid to be ground. As a result, the solids concentration of the slurry at discharge point B increases and decreases in accordance with the changes in hardness.

It has been determined that when the mill 10 is operating at maximum efliciency, the slurry within the mill is of a density whereby grinding eificiency is increased and the apparent density of the discharged slurry has the value p It is noted that during the production period t to t hard solids 12 were admitted to the mill causing the apparent density of the slurry to drop below the desired value p by an amount On the other hand since the hardness of the solid feed decreased during the period t to t;.,, the mill, now being capable of reducing the material beyond the required size, discharged slurry having an error in density substantially of the same magnitude but in the opposite direction.

Referring now to FIG. 3, the present invention provides control for adjustment of the rate of inflow of liquid 14' to the mill in accordance with the density measurements of a transducer 22 installed in the effluent line as close as physically possible to the discharge point B. Transducer 22 may be any of the well-known electrical, mechanical or electromechanical devices for producing a signal in response to changes in slurry density. But, a preferred form comprises a radiation absorption gauge which produces a density-functional electrical signal Ep in a manner described hereinafter. An error sensor 24 receives the output of the transducer 22 and continuously compares this signal with another electrical signal Ep from a specification generator 26 representative of the desired density p This latter signal determines the set point of the control system and may be developed by a potentiometric voltage dividing network. Error sensor 24 delivers an error signal Ep of a magnitude and polarity in proportion to the difference between E0,, and Ep Error signal E is applied to a controller 28 which actuates a two-phase Servomotor 30 through a delay unit 32. Servomotor 30 is connected by suitable speed reduction means 34 to the liquid valve 18'.

Controllers found by applicant to have suitable characteristics for effectively carrying out the operational details incidental to the practice of the invention may be classified into .two general types. One type is referred to as a continuous or integrating controller such as is described in a co-pending application Ser. No. 657,434, filed May 6, 1957, by Richard F. Warren, now US. Patent 2,999,406, issued Sept. 12, 1961. The other is a reset type controller described in U.S. Letters Patent No. 2,895,888 filed Oct. 7, 1957 by Donald E. Varner.

In the operation of the present invention a decrease in slurry density is detected at 22. In response to the error signal Ep controller 28 directs Servomotor 30 to rotate valve 18' in a direction to decrease the flow of liquid into the mill 10'. Servomotor 30 operates for a time proportional to the magnitude of error detected at 24. The decrease in liquid enables the required size reduction to take place so that the slurry density more nearly approaches the desired value p An increase in slurry density caused by soft solids entering the mill 10' is similarly detected. Now, however, controller 28 responds by reversing the servomotor 30 to increase the liquid flow into the mill 10'. Since the amount of liquid required for grinding is less due to the inherent increased grindability of the soft solids, enough liquid is admitted to maintain the density at the desired value. The purpose of the delay 32 is to allow corrective adjustment of the valve 18' only after the result of a previous adjustment has been examined by the transducer 22. Alternatively, a half-size correction may be taken twice as often. Delay unit 32. compensates for transportation lag and eliminates controller overshoot which might otherwise prevail. The amount of delay may be provided by a timer and is equivalent to the time required for the transport of solids 12 through the mill 10'.

FIG. 4 graphically depicts the results of density control.

The present invention may be advantageously applied to closed circuit grinding processes with the modifications shown in FIG. 5. Referring to FIG. 5, the slurry discharge of a ball mill 40 in an outlet conduit 42 is transmitted to a classifier 44 which separates over-size particles from the slurry by well known means. To facilitate the size classification of the solids, fresh Wash water is admitted by a valve 46 to the classifier 44. From classifier 44, the coarse rake product is recirculated through a conduit 48 to the ball mill 40 until the required size reduction is accomplished. I-Ieretofore, the mill 40 has had to cope with large recirculating loads when grinding hard material; accordingly, the amount of new feed at 50 had to be manually reduced to prevent choking the mill 40. As a result, the throughput of the mill has not been maximized. And, in general, the system has not been operated in the most efiicient manner possible.

The present invention provides for control of the process in accordance with density measurements of transducers 52 and 54, and return sand level measurements of a gauge 56. The density measurements of transducer 52 mounted on conduit 42 control the amount of water admitted to the mill input through a valve 58 to maintain the mill discharge at the desired density. The density control system indicated at 60 is similar to that described hereinabove with reference to FIG. 3 so further explanation is deemed unnecessary. The transducer 52 is located in the outlet conduit 42, and transducer 54 and level gauge 56 are located on conduit 48 near the input of the mill 40.

In the illustrated embodiment, the return sands from classifier 44 flow at a substantially constant velocity toward the input of mill 40. By measuring both the level and the density of the return sands in the conduit 48, as described hereinafter, it is possible to obtain an indication of the mass flow per unit time m/t of the return sands. To this end, computer 62 is connected to the density transducer 54 and the level gauge 56 to combine the output signals thereof, and thus derive a signal proportional to m/ t. The circuit required at 6-2 is well known to those skilled in the art and may be that described in US. Letters Patent No. 2,880,935, issued Apr. 7, 1959 to T. J. Johnson. The mass flow rate functional signal from the computer 62 provides the input to a control system 64 which in turn adjusts a variable speed drive means 66 for a feed conveyor 68. The speed of feed conveyor 68 determines the rate at which new solids are added to the mill 40.

Operation is similar to that described hereinabove. Water feed to the mill 40 is adjusted at 58 to continuously maintain the slurry density at the specification set into the control system 60. In this manner, the amount of coarse material fed back from the classifier 44 is less for a given rate of solid material feed. Now, the speed of conveyor 68 can be increased by the controler 64 until the combined rate of new solids addition plus the rate of return sand flow approaches the choking capacity of the mill. The specification set into controller 64 should be representative of a quantity of solid input flow somewhat less than the choking capacity of the mill. If the hardness of the new solids being fed to the mill increases, the recirculating load from the classifier also increases. Controller 64 responds to an increase in m/t of the recirculating load by reducing the speed of conveyor 68, thereby decreasing the rate of new solids being fed to the mill 40. With this scheme, the mill may be operated at near choking capacity, but with a considerable increase in desirable throughput.

In addition the present invention may be used to provide rigorous control of the particle size of slurry produced by a closed grinding circuit. As the water added to the mill 40 controls the density of the slurry for optimum grinding, the wash water added to the classifier 44 controls the size-of-grind in the overflow line 70. If a drag classifier is used at 44, more wash water flushes through the classifier 44, the size of the solids in the overflow 70 decreases. Conversely, if the rate of wash water feed is reduced, more of the larger particles exit the classifier 44. However, if a cyclone classifier is used, the converse is true. By installing the density measuring and controlling system of the present invention at 72 in the classifier overflow 70, it is possible to adjust the valve 46 so that the overflow of classifier 44 comprises solids of a uniform specific size.

The output of the clasifier 44 may go to flotation stages or if the mill 40 is a primary grinder the classifier output may be transmitted to one or more secondary grinding circuits each of which may be advantageously controlled by the present invention in a manner similar to that described hereinabove.

Heretofore, it has been diflicult to measure the mass per unit time of the return sands in conduit 48. Due to the presence of various iron compounds in the ground ore, it has been impossible to use commercially available flow meters. A construction for a mass flow meter useful in the preferred embodiment shown in FIG. 5 is illustrated in FIG. 6. Referring now to FIG. 6, conduit 48 comprises a chute for transporting the return sands from the classifier to the mill 40. The density transducer 54 comprises a radiation source unit 54a and a radiation detector unit 54b mounted at opposite sides of the chute 48. The construction of these elements will become apparent hereinafter in the description of FIGS. 8 and 9. These units are preferably mounted near the bottom of chute 48 so that regardless of a decrease in the recirculating load a certain quantity of the return sands are always located therebetween. Near the top of the chute 48 is mounted another radiation source unit 56a and an elongated radiation detector unit 56b comprising the level gauge 56, The detector 5617 may be a Geiger-Muller tube.

Briefly, a beam of radiation is directed toward the detector 56b. As the level of the return sands in the chute 48 increases, the amount of detected radiation decreases and conversely. The output of the detector will be proportional to the height h of the return sands in the chute. If in the normal operation of a given grinding process, the level changes very little, the illustrated horizontal mounting of the detector 56b will be quite adequate. However, if there is a large variation in sand level, it may be desirable to utilize a vertical mounting of the same to accommodate a wide band measurement. A more detailed description of the level gauge 56 may he had by referring to a copending application Ser. No. 730,347, filed Apr. 23, 1958, by Jack G. Crump, now US. Patent 3,001,076, issued Sept. 19, 1961.

The cement grinding process of FIG. 7 is one example of an industrial grinding process wherein the utilitarian aspects of the present invention are immediately apparent. Referring to FIG. 7, limestone, clay and other ingredients at 74 are fed into a compeb mill 76 which grinds the raw materials to form cement slurry. The slurry discharged from the compeb mill 76 passes through an outlet conduit 78, a mill basin 88, correction tanks 82, and a filter 84. Slurry precipitate from the filter 84 continues to kilns for burning into clinkers. Aqueous filtrate from the filter 84 is fed back to the input of the compeb mill 76 via an adjustable valve 85 to facilitate the grinding of new raw materials. Depending on the hardness and moisture content of the raw materials 74, the slurry discharge of the compeb mill 76 may vary between 30% and 40% water. It has been found that a moisture content of 34% is economically preferable. Slurries having less moisture are not easily ground by the compeb mill 76; and, slurries having a higher water content require large amounts of heat to evaporate the excess moisture. The control system of the present invention is applied to this process to maintain the mill discharge at a density indicative of the preferred moisture content. A discussion of the correlation of moisture content to apparent density will be taken up hereinafter.

In the preferred form, the density transducer comprises a source of penetrating radiation 88 and a radiation detector positioned adjacent opposite sides of the outlet conduit 78 near the discharge point of the compeb mill 76. In accordance with known behavior, the absorption of radiation by the conduit 78 and the slurry carried therein is primarily a function of the energy of the source 88, the composition of the irradiated substances and the effective mass cross-section of the absorbing medium. In the present application, the only variable is the latter quantity. Since mass cross-section is equal to the product of density and thickness, and since the thickness in the present case is constant, being equivalent to the distance between the source 88 and detector 90, only changes in the density of the absorbing medium, viz., the slurry and conduit 78, affect the radiation detected at 90. Moreover, since the density of the conduit 78 may be considered constant, the apparatus is sensitive only to the apparent density pm of the liquid-solid mixture carried by the conduit 78.

The quantity pm may be calculated thus:

where =density of the liquid component =density of the solid component W =weight of liquid in volume V of mixture w =weight of solid in volume V of mixture Inasmuch as the temperature and pressure throughout the system do not vary, the quantities p and p are considered to remain substantially constant. And, since the volume is easily computed. It may be apparent that this fraction multiplied by one hundred comprises the percent moisture in the examined mixture. Accordingly, the density-functional output of the detector 90 is coupled to a measuring circuit 92 for translation to percent moisture of the slurry. The measuring circuit described in US. Letters Patent No. 2,790,945, issued Apr. 30, 1957, to H. R. Chope, is suitable for use at 92. The output of the measuring circuit 92 on line 94 may not only be read out on an indicator such as a chart recorder 96, but may also be used to control the adjustment of valve 86.

The control system enclosedin the dotted outline 98 is substantially identical to the reset control apparatus described hereinabove with respect to FIG. 3. The set point of control system 98 is determined by a moisture target setter 100 which is suitably calibrated in units of percent moisture for the convenience of the operator. A signal indicative of the desired moisture content manually set at 100 is applied to an error sensor 102 together with the output of the measuring circuit 92. A controller 104 operates on the error difference between the desired moisture and the measured moisture to adjust valve 86 through a delay circuit 106, servomotor i108 and speed reducing gear box 109. In some cases, it may be desirable to use pneumatic rather than mechanical actuation of the valve 86. Those skilled in the art will be aware of the modifications necessary for such a change.

In the operation of the preferred embodiment, if the moisture content of the cement slurry exceeds the value set at 100, servomotor 108 rotates in a direction to decrease the flow of filtrate through valve 86 into the compeb mill 76 and for a time proportional to the magnitude of the sensed error. Conversely, if the slurry moisture content is less than the desired value, valve 86 is opened. It should be noted that, as in the operation of the foregoing embodiments, delay 106 confines corrective adjustment of valve 86 to periodic intervals determined by the material transport lag of the process. Accordingly; if the amount of filtrate being added to or subtracted from the mill input does not bring the cement slurry moisture on target, further corrective action is taken by the control system 98 only after a period determined by delay unit 106.

With reference now to FIGS. 8 and 9, detailed mechanical construction of a housing for the radiation source and detector units shown in FIGS. 6 and 9 is shown merely as an example. Referring first to FIG. 8, a short section of stainless steel pipe 110 is provided at either end with a flange 112 and carries about its midsection a substantially cubical housing 114 having a pair of outwardly directed frusto-conical end pieces 116, L18 bolted to opposite faces thereof. Housing 114 is constructed in two pieces which may be clamped about the pipe 110 and secured by elongated bolts as at 120.

In referring to FIG. 8, the reasons for the chosen design become apparent. In one half of the housing 114 is mounted a source holder 122 adapted to carry a source capsule 124. The source capsule 124 contains a radioisotope such as cesium 137 and is preferably mounted as close as possible to the pipe 110. The end piece L18 covers the source holder 122 and serves to reduce external radiations from the rear of source capsule 124. The radiation from capsule 124 is detected by an ionization chamber 126 mounted in the other half of the housing 114 by bonding to an annular insulator 128 having a step 130 formed thereon. The radiation transparent window 126a of the chamber 126 is positioned adjacent the pipe 110. The wall thickness of the pipe 110 may be milled at 110a to reduce the mass traversed by the radiation beam, thereby increasing the sensitivity of the measuring instrument The illustrated density transducer comprises a compact instrument which may be readily inserted in the slurry discharge conduit 78 of FIG. 7. In particular, the distance between the source and detector elements is rigorously maintained by the rugged construction. Thus, the volume V of slurry examined remains constant and the moisture calibration of measuring circuit 92 is maintained. The illustrated construction may also be incorporated with success into the embodiments of FIGS. 3 and 5. The unit is readily adapted for mounting on the chute 48 shown in FIG. 6 by eliminating the pipe 110 and the elongated bolts 120. But the present invention should not be restricted by the exemplary construction shown.

By referring to FIG. 10, the response of the preferred density transducer is graphically illustrated by a curve 132. Due to the nature of the radiation absorption phenomenon, the response of the chamber 126 is large when the slurry contains little solid material and decreases exponentially as the solids content increases. At an appai'ent density p where the moisture content of the slurry is of the desired value, it is noted that the slope of the curve 132 is quite large. Moreover, the response curve 132 is linear in the range of slurry densities p;, to p normally encountered in the cement process.

Modifications and changes from the embodiments of the invention shown and described herein may be made without departing from the spirit of the invention, and such modifications and changes are intended to be embraced within the scope of the appended claims.

What is claimed is:

1. The method of controlling the solids concentration and throughput of a slurry produced by a closed circuit wet-grinding mill wherein new solids of varying grindability and a liquid are added at individually controllable rates to the input of said mill to form said slurry and underflow returning from a classifier is added to said input, said method comprising the steps of measuring the density of said slurry at the output of said mill, varying the rate at which said liquid is added in accordance with said density measurements, measuring the mass per unit time of said classifier underflow, and varying the rate at which said new solids are added in accordance with said mass per unit time measurement.

2. The method of controlling the solids concentration and throughput of a slurry produced by a closed circuit wet-grinding mill wherein new solids of varying grinde ability and a liquid are added at individually controllable rates to the input of said mill to form said slurry and underflow returning at a substantially constant velocity from a classifier is added to said input, said method comprising the steps of measuring the density of said slurry at the output of said mill, varying the rate at which said liquid is added in accordance with said density measurements, establishing a path of predetermined cross-section for said classifier underflow, measuring the density of said classifier underflow, measuring the cross-sectional area of said underflow in said established path, combining said density and area measurements to derive a signal proportional to the mass per unit time of said underflow added to said mill, and varying the rate at which new solids are added to said mill in accordance with said derived signal. a

3. Apparatus for controlling the solids concentration and throughput of a slurry produced by a closed circuit Wet-grinding mill wherein new solids of varying grindability and a liquid are added at individually controllable rates to the input of said mill to form said slurry and underflow returning from a classifier is added to said input, means for measuring the density of said slurry at the output of said mill, means for varying the rate at which said liquid is added in accordance with said density measurements, means for measuring the mass per unit time of said classifier underflow, and means for varying the rate at which said new solids are added in accordance with said mass per unit time measurement.

4. Apparatus for controlling the solids concentration and throughput of a slurry produced by a closed circuit wet-grinding mill wherein new solids of varying grindability and a liquid are added at individually controllable rates to the input of said mill to form said slurry and underflow returning at a substantially constant velocity from a classifier is added to said input, means for measuring the density of said slurry at the output of said mill, means for varying the rate at which said liquid is added in accordance with said density measurements, means for establishing a path of predetermined cross-section for said classifier underflow, means for measuring the density of said classifier underflow, means for measuring the crosssectional area of said underflow in said established path, means for combining said density and area measurements to derive a signal proportional to the mass per unit time of said underflow added to said mill, and means for varying the rate at which new solids are added to said mill in accordance with said derived signal.

No references cited.

WILLIAM W. DYER, IR., Primary Examiner.

G. A. DOST, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,352,499 Dated November 14, 1967 Inventor(3) R. L. Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 62, "controler" should read --controller-- Column 6, lines 29 to 32, the portion of the denominator on the right hand side of the formula which reads "P i W 1 s should read Signed and sealed this 1st day of. June 1971.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. Attesting Officer WILLIAM E. SCHUYIER, JR. Commissioner of Patents F ORM PO-1050 (10-69) USCOMM-DC 603764 6 0,5. GOVERIIIIUIT "nmuo ornc: mu 0-306-3: 

1. THE METHOD OF CONTROLLING THE SOLIDS CONCENTRATION AND THROUGHPUT OF A SLURRY PRODUCED BY A CLOSED CIRCUIT WET-GRINDING MILL WHEREIN NEW SOLIDS OF VARYING GRINDABILITY AND A LIQUID ARE ADDED AT INDIVIDUALLY CONTROLLABLE RATES TO THE INPUT OF SAID MILL TO FORM SAID SLURRY AND UNDERFLOW RETURNING FROM A CLASSIFIER IS ADDED TO SAID INPUT, SAID METHOD COMPRISING THE STEPS OF MEASURING THE DENSITY OF SAID SLURRY AT THE OUTPUT OF SAID MILL, VARYING THE RATE AT WHICH SAID LIQUID IS ADDED IN ACCORDANCE WITH SAID DENSITY MEASUREMENTS, MEASURING THE MASS PER UNIT TIME OF SAID CLASSIFIER UNDERFLOW, AND VARYING THE RATE AT WHICH SAID NEW SOLIDS ARE ADDED IN ACCORDANCE WITH SAID MASS PER UNIT TIME MEASUREMENT. 